Considerable increases in the utilization of electrical energy over the last few years has occurred as more electrical equipment is being used or installed by an increasing number of users or customers. Along with the demand for electrical energy, the demand for its quality has also increased. No longer is variation in the voltage level, frequency or phase acceptable. Particularly, in applications such as hospitals, process intensive industries, data centers, etc, the quality of the provided electrical energy is especially important.
Uninterruptible Power Supplies, which provide a source of power during interruptions in main electrical generation, generally accept two inputs; one for its rectifier and the other for the bypass circuit. If the customer is willing to have a 4 wire output the customer, typically, has to supply 4 wires also for the bypass circuit. The fourth wire or neutral wire for the bypass will be permanently connected to the output neutral. When the UPS is working in normal double conversion mode, the inverter has to generate the neutral and should ensure its potential is stable with respect to a reference value (e.g., a ground or zero voltage level). This can be achieved by either taking a four wire input for the rectifier and use the 4th wire (i.e., rectifier input neutral) as a load neutral or take a three wire input for the rectifier and by well-known technique generate a 4th wire in inverter.
For the UPS with output isolation transformers, the three-wire input and four-wire output configuration is solved with the output transformer. This provides a galvanic isolation, but considerably increases the weight of the UPS and often affects its footprint and Cost. Transformerless UPS designs are known in the art. See for example, “Transformerless Four-Wire PWM Rectifier and its Application to AC-DC-AC Converters,” Fraser, M. E., et al., IEE Proceedings on lone no. 19952278, 1995).
However, conventional designs suffer from high input current harmonic distortion at non-linear and unbalanced loads.
Hence, there is a need in the industry for a transformerless UPS having at least a low input total harmonic distortion.